Microwave radiometry for noninvasive monitoring of brain temperature

Microwave radiometry is a passive and noninvasive technique that is able to measure deep tissue temperature and track changes in thermal profiles in tissue up to 5 cm below the surface over several hours. These characteristics make microwave radiometry a suitable technique to monitor brain temperature during extended hypothermic surgeries, and thus avoid potential complications that result from poorly controlled low temperature levels and return to normothermia. This chapter addresses all development stages of a radiometric brain thermometer including: radiometer electronics; antenna design and fabrication; power to temperature calibration algorithm; multilayer head phantom model with variable temperature compartments; experimental validation of sensor performance; and initial clinical implementation. In particular, a radiometric antenna is described based on a log-spiral design optimized in silico to receive energy from deep brain. The prototype is tested using a realistic head phantom that consists of an anatomical human skull with separate brain and scalp compartments in which tissue-equivalent fluid phantoms circulate at independent temperatures (32 °C for scalp and 37 °C for brain). Experimental data shows that the calculated radiometric brain temperature tracks within 0.4 °C the measured brain phantom temperature over a 4.6 h experiment, when the brain phantom is lowered 10 °C and then returned to original temperature. A clinical case confirms the ability to noninvasively monitor temperature in deep brain using microwave radiometry, with radiometric measurements that closely track changes in core temperature as measured in the nasopharynx. Both simulated and experimental results demonstrate that a 1.1–1.6 GHz radiometric sensor with 2.5 cm diameter is an appropriate tool for noninvasive monitoring of deep brain temperature.